The lac repressor and the RNA polymerase can be cross-linked to DNA that contains bromouracil in place of thymine. Ultraviolet light irradiation knocks off the bromine and creates a free radical which reacts with the protein. We hope to identify which amino acids in the lac repressor are close to which bases in the operator, in order to form a three-dimensional picture of that specific interaction. We wish to identify which subunits of the RNA polymerase interact with DNA and to determine the DNA regions that those subunits contact.